With the recent control of emissions of volatile organic solvents, previously known organic-solvent-based paints, ink, coating agents, adhesives, sealing agents, primers, and agents for treating fiber products and paper products are becoming replaced with aqueous agents, high-solid agents, or powder agents. In particular, the production of aqueous agents using aqueous dispersions ensure good workability and an improved work environment, and thus are considered most versatile and promising. Further, in forming the aqueous dispersion, it is more preferable to use a binder component mainly containing a biodegradable resin in terms of preventing environmental pollution after disposing of the product. It is further preferable to use a binder component that is produced by using a biomass-derived component as a raw material, such as an animal-derived or plant-derived biomass component, in terms of reducing carbon dioxide emissions, compared with a binder component produced from a fossil fuel-derived component.
Polylactic resins are mainly made of a plant-derived component that can be produced from a lactic acid and/or a lactide that can be produced from a plant-derived raw material, such as corn or potato. Polylactic resins are biodegradable and are thus decomposed into water and carbon dioxide in the soil or sea within a few years. Therefore, polylactic resins cause a relatively small environmental burden when emitted into the environment. Thus, if a polylactic resin can be made into an aqueous dispersion, it becomes possible to produce a useful binder component that has biodegradability and is produced from a biomass-derived component. Such a binder component can be expected to be used as a paint, ink, a coating agent, an adhesive, glue, a sealing agent, a primer, an agent for treating a fiber product or a paper product, or the like.
Patent Documents 1 to 5 disclose examples of using a binder component obtained by dispersing a resin containing a polylactic acid segment in water. Patent Documents 1 and 2 disclose polylactic acid aqueous dispersion forcibly emulsified by using an emulsifier. Patent Document 3, which also discloses an aqueous polylactic acid forcibly emulsified by using an emulsifier, discloses that a hydrophilic group may be introduced into the resin, and in particular, sodium salt of 5-sulfoisophthalic acid or sodium salt of dimethyl 5-sulfoisophthalate are preferable in terms of excellent emulsifiability. Patent Document 4 discloses copolymerized polyurethane that has a polylactic acid segment and a segment containing a sulfonic acid metal salt group in the molecule, and has a self-emulsifying function that enables formation of a stable aqueous emulsion without an emulsifier. Further, Patent Document 5 discloses a method for producing self-water dispersible particles by reacting a lactic acid polymer having a hydroxyl group with a polycarboxylic acid or an acid anhydride thereof, thus dissolving the lactic acid polymer in an organic solvent, and then adding a base and water thereto to cause phase-transfer emulsification.